Petrologic characterization of pelitic schists in the western metamorphic belt, coast plutonic-metamorphic complex, near Juneau, southeastern Alaska

Abstract

The metamorphic rocks exposed near Juneau, Alaska, are part of the western metamorphic belt of the Coast plutonic-metamorphic complex of western Canada and southeastern Alaska. This belt underwent a complex history of deformation, metamorphism, and plutonism that ranges in age from about 120 to 50 Ma as a result of tectonic overlap and (or) compressional thickening of crustal rocks during collision of the Alexander and Stikinia terranes. Most of the schists near Juneau were metamorphosed during the M5 metamorphic event about 70 m.y. ago and are part of a belt of schists of similar metamorphic age that extends for at least 400 km to the southeast. Distribution of pelitic mineral isograds, systematic changes in mineral assemblages, and silicate geothermometry of the Juneau schists indicate an inverted metamorphic gradient over a distance of about 5 km. Changes in mineral assemblages across isograds separating the metamorphic zones are in general agreement with discontinuous reactions in the ideal SO2-A1203-K,OFe0-Mg0-H20 system. Biotite and staurolite in low variance assemblages show changes in Mg/(Mg+Fe) ratios with metamorphic grade that are generally consistent with predicted changes in the ideal system, although exceptions exist. The observed metamorphic mineral assemblages and previously determined peak temperatures and pressures (530°C for the garnet zone to about 705°C for the upper kyanite-biotite zone, and 9 to 11 kbar) are consistent with recently published petrogenetic grids for pelitic rocks. Maximum mole fraction of H20 as constrained by distribution of species in the C-0-H system for graphitic schists ranges from about 0.93 for the garnet zone to about 0.90 for the upper kyanite-biotite zone. Metamorphic mineral growth was synchronous with the third of four recognized folding events. Manuscript approved for publication, October 14, 1993. INTRODUCTION The metamorphic rocks exposed near Juneau, Alaska are part of the western metamorphic belt of the informally named Coast plutonic-metamorphic complex (Brew and Ford, 1984) of western Canada and southeastern Alaska (fig. 1). The western metamorphic belt and associated plutons developed as a result of tectonic overlap and (or) compressional thickening of crustal rocks during the collision of two large allochthonous terranes-the Alexander terrane on the west and the Stikinia terrane to the east (Monger and others, 1982; Brew and Ford, 1983). The western metamorphic belt is one of the major metamorphic features of southeastern Alaska (Brew and others, 1992). The western metamorphic belt ranges from a few kilometers to several tens of kilometers wide and is characterized in general by Barrovian metamorphism. The metamorphic grade increases to the northeast, with the highest grade adjacent to the plutonic part of the Coast plutonic-metamorphic complex. At the latitude of Juneau the western metamorphic belt is about 30 km wide. The mineral isograds, systematic changes in mineral assemblages, and structural relations indicate an inverted metamorphic gradient along the easternmost part of the belt, where the metamorphic grade goes abruptly from pumpellyite-actinolite facies to kyaniteand sillimanitebearing amphibolite facies in less than 5 km across strike. The development of the inverted metamorphic sequence and the geothermometry and geobarometry associated with its formation was recently discussed by Himmelberg and others (1991). This report extends that work to include more detailed data on mineral assemblages, metamorphic reactions, structural relations, age constraints, and mineral chemistry that were not included in the earlier paper. These data and discussions provide a more complete characterization of the western metamorphic belt in the Juneau area and, along with the earlier paper, will serve as a foundation to continually evaluate the evolution of western metamorphic belt as other data are obtained. PETROLOGIC CHARACTERIZATION OF PELITIC SCHISTS, NEAR JUNEAU, ALASKA